What is sadness? I'm not asking what causes it, but what it is.?

Although depression is often thought of a being an extreme state of sadness, there is a vast difference between clinical depression and sadness. Sadness is a part of being human, a natural reaction to painful circumstances. All of us will experience sadness at some point in our lives. Depression, however, is a physical illness with many more symptoms than an unhappy mood. The person with clinical depression finds that there is not always a logical reason for his dark feelings. Exhortations from well-meaning friends and family for him to "snap out of it" provide only frustration for he can no more "snap out of it" than the diabetic can will his pancreas to produce more insulin. Sadness is a transient feeling that passes as a person comes to term with his troubles. Depression can linger for weeks, months or even years. The sad person feels bad, but continues to cope with living. A person with clinical depression may feel overwhelmed and hopeless.

Melancholia is classified as a type of depression in the DSM IV and is referred to as Dysthymia:

Dysthymia is a mood disorder that falls within the depression spectrum. It is considered a chronic depression, but with less severity than a major depression. This disorder tends to be a chronic, long-lasting illness.

Sadness can only be described as being blue all day long. You just want to cry and cry all day.

A feeling, a mood. More specifically, an acute neurological condition.

Melancholia.

an emotion. all emotions are just chemical changes in your brain brought on by external or internal input. mostly it has to do with your personality and what you find sad.

...no happiness in the moment...